1. Field of the Invention
This invention relates to a stencil printer, and more particularly to a stencil printer in which a stencil master can be constantly held in a predetermined position on the printing drum and printing papers and the stencil master are prevented from being wrinkled.
2. Description of the Related Art
In a stencil printer, a stencil master is wrapped around a printing drum and the printing drum is rotated. A press roller in contact with the stencil master on the printing drum is rotated together with the printing drum and a printing paper is fed between the stencil master and the press roller by a paper feed mechanism. The printing paper is conveyed pinched between the stencil master and the press roller and ink supplied inside the printing drum is transferred to the printing paper through perforations in the stencil master.
In such a stencil printer, the printing paper must be fed between the printing drum and the press roller at a timing such that the printing paper exactly overlaps with the stencil master in a predetermined position relative to the stencil master. For this purpose, adjustment for ensuring that the printing paper exactly overlaps with the stencil master in a predetermined position is carried out upon starting of printing.
In conventional stencil printers, the paper feed mechanism generally comprises primary and secondary paper feed sections which are driven by the printing drum by way of a transmission mechanism such as those including gears.
The primary and secondary paper feed sections in the conventional stencil printers will be described hereinbelow.
In the primary paper feed section, printing papers stacked on a paper feed table are fed one by one for one rotation of the printing drum by a pickup roller and a scraper and conveyed to the secondary paper feed section. The pickup roller and the scraper are intermittently rotated by a main motor, which drives the printing drum, by way of a paper feed clutch which is selectively engaged and disengaged on the basis of a signal from a drum position sensor which detects the angular position of the printing drum. The pickup roller and the scraper are provided with a one-way clutch and the paper feed clutch is disengaged after the primary paper feed section delivers the leading end of the printing paper to the secondary paper feed section so that the pickup roller and the scraper run free and back tension is reduced.
In the secondary paper feed section, the leading end of the printing paper fed by the pickup roller and the scraper abuts against a guide roller or a timing roller near the contact line of the guide roller and the timing roller (will be referred to as "the conveyor roller pair", hereinbelow) which are stopped and the printing paper sags. Then the conveyor roller pair are started when the printing drum is in a predetermined phase of rotation. Each roller of the conveyor roller pair is provided with a gear on each end of its shaft and the gears on the shafts of the rollers on each end thereof are in mesh with each other. The guide roller is caused to make several rotations in one direction per one rotation of the printing drum by the main motor by way of a transmission mechanism comprising gears or an endless belt, a cam, a sector gear, a one-way clutch and the like. The timing roller is rotated in the direction opposite to the guide roller driven by the guide roller. The timing roller is moved away from the guide roller after the guide roller is stopped by a mechanism including, for instance, a cam, a cam follower, a link member and a resilient member. Further, the timing roller is provided with a spring or an electromagnetic brake on one end of its shaft so that the timing roller is stopped as soon as it is disengaged from the guide roller without overshooting under inertia.
The printing paper conveyed by the conveyor roller pair is fed between the printing drum and the press roller pressed against the printing drum at a predetermined pressure and ink supplied from an ink supply section disposed inside the printing drum is transferred to the printing paper through image-wise perforations in the stencil master while the printing paper is conveyed pinched by the printing drum and the press roller.
However since the timing roller is moved away from the guide roller upon end of conveyance of the printing paper as described above, the printing paper can run obliquely when it is delivered to the printing drum and the press roller, which can wrinkle the printing paper during subsequent conveyance of the printing paper by the printing drum and the press roller. Further there has been a problem that the printing paper is wrinkled by tensile force momentarily generated when the trailing end portion of the printing paper is caught by the pickup roller and the scraper. Wrinkle in the printing paper can wrinkle the stencil master and at the same time can shift the stencil master from the predetermined position on the printing drum.